<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title></title>
        <style type="text/css">
            * {
                margin: 0;
                padding: 0;
            }

            li {
                list-style-type: none;
            }

            a {
                text-decoration: none;
                font-size: 14px;
            }

            .nav {
                margin: 100px;
            }

            .nav>li {
                position: relative;
                float: left;
                width: 80px;
                height: 41px;
                text-align: center;
            }

            .nav li a {
                display: block;
                width: 100%;
                height: 100%;
                line-height: 41px;
                color: #333;
            }

            .nav>li>a:hover {
                background-color: gray;
            }

            .nav ul {
                display: none;
                position: absolute;
                top: 41px;
                left: 0;
                width: 100%;
                border-left: 1px solid #FECC5B;
                border-right: 1px solid #FECC5B;
            }

            .nav ul li {
                border-bottom: 1px solid #FECC5B;
            }

            .nav ul li a:hover {
                background-color: pink;
            }
        </style>
    </head>
    <body>
        <ul class="nav">
            <li>
                <a href="#">微博</a>
                <ul>
                    <li>
                        <a href="">私信</a>
                    </li>
                    <li>
                        <a href="">评论</a>
                    </li>
                    <li>
                        <a href="">@我</a>
                    </li>
                </ul>
            </li>
            <li>
                <a href="#">微博</a>
                <ul>
                    <li>
                        <a href="">私信</a>
                    </li>
                    <li>
                        <a href="">评论</a>
                    </li>
                    <li>
                        <a href="">@我</a>
                    </li>
                </ul>
            </li>
            <li>
                <a href="#">微博</a>
                <ul>
                    <li>
                        <a href="">私信</a>
                    </li>
                    <li>
                        <a href="">评论</a>
                    </li>
                    <li>
                        <a href="">@我</a>
                    </li>
                </ul>
            </li>
            <li>
                <a href="#">微博</a>
                <ul>
                    <li>
                        <a href="">私信</a>
                    </li>
                    <li>
                        <a href="">评论</a>
                    </li>
                    <li>
                        <a href="">@我</a>
                    </li>
                </ul>
            </li>
        </ul>
    </body>
    <script type="text/javascript">
        var nav = document.querySelector('.nav');
        var lis = nav.children;
        for (i = 0; i < lis.length; i++) {
            lis[i].onmouseover = function() {
                this.children[1].style.display = 'block';
            }
            lis[i].onmouseout = function() {
                this.children[1].style.display = 'none';
            }
        }
    </script>
</html>
